Apparatus and method for improving coupling across plane discontinuities on circuit boards

ABSTRACT

The invention relates to an apparatus and method for improving coupling across plane discontinuities on circuit boards. A circuit board includes a discontinuity, e.g., a split, slot, or cutout, formed on a voltage reference plane. A conductive layer overlies the discontinuity. The conductive layer has a first portion connected to the underlying reference plane and a second portion spanning the discontinuity. The first portion is connected to the reference plane using a slot or vias. And the conductive layer has a third portion extending over the reference plane but remaining disconnected from it. The conductive layer might be graphite or carbon black.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to circuit boards (CBs), e.g., printed circuitboards, imprinted boards, and any other support substrate. Moreparticularly, to an apparatus and method for improving coupling betweensignals routed across plane discontinuities on CBs.

2. Description of the Related Art

CBs are typically manufactured with a plurality of layers, each layerbeing permanently affixed to an adjoining layer through a structural,non-conductive material. FIG. 1 is an example of a four layer CB 100.Referring to FIG. 1, the first and fourth layers 112 and 124,respectively, are signal trace layers on which signal lines are routed.The second and third layers 116 and 120, respectively, are voltagereference layers, e.g., the second layer 116 is a power layer and thethird layer 120 is a ground layer. The power layer 116 and ground layer120 are affixed to a core 118 comprised of a fiberglass mesh materialsuch as FR4. The first signal layer 112 and the power layer 116 and thesecond signal layer 124 and the ground layer 120 each sandwich a preimpregnated epoxy material 114, commonly referred to as pre-preg.

Signal lines are primarily routed on the first and fourth layers 112 and124, respectively. Oftentimes, signal lines must be routed from thefirst layer 112 to the fourth layer 124 through the reference layers 116and 120. When this occurs, the power layer 116 and the ground layer 120must be split, cutout, or slotted (the result is collectively termed adiscontinuity) to avoid a short circuit between the signal lines beingcross-routed and the reference layers. Signals routed around thecross-routed signal must necessarily be laid out across thediscontinuity.

FIG. 2A is a cross sectional view of an exemplary CB 200 with adiscontinuity 202. FIG. 2B is a top view of the CB 200. Referring toFIGS. 2A-B, the CB 200 includes a ground plane (or layer) 204 andvoltage planes (or layers) 206 and 208. A discontinuity 202 existsbetween voltage planes 206 and 208. A signal 210 bridges or spans thediscontinuity 202 as most clearly shown in FIG. 2B.

High-speed signals that span discontinuities in adjacent referenceplanes, like signal 210, generate electromagnetic (EM) radiation becauseof the electrical break caused by the plane discontinuity. This EMradiation adversely affects electromagnetic containment (EMC) and signalintegrity (SI). For one, the discontinuity increases the electricalground path increasing loop inductance. And the larger inductance mightcause signal distortion and phase shifts.

To avoid these issues, CB designers avoid routing high-speed signalsover discontinuities. But these constraints are often difficult tomaintain as CB real estate shrinks or as signal density increases.Another way CB designers avoid these problems is to use stitchingcapacitors across discontinuities, e.g., stitching capacitor 212. Thestitching capacitor 212 electrically couples the plane 208 to the plane206 through vias 214. The stitching capacitor 212 provides alternatingcurrent (AC) coupling that reduces EM radiation at the discontinuityreducing, in turn, adverse EMC and SI effects. The addition of stitchingcapacitors, however, is costly. One stitching capacitor is required foreach signal crossing a discontinuity. Thus, the CB component countincreases, increasing cost. More components require additional CB realestate, also increasing cost.

Accordingly, a need remains for an apparatus and method of improvingcoupling across plane discontinuities on circuit boards.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of theinvention will become more readily apparent from the detaileddescription of an embodiment that references the following drawings.

FIG. 1 is a cross sectional view of a CB.

FIG. 2A is a cross sectional view of another CB.

FIG. 2B a top view of the CB shown in FIG. 2A.

FIG. 3 is a cross sectional view of a CB according to the presentinvention.

FIGS. 4A-C are diagrams of the CB discontinuity shown in FIG. 3.

FIG. 5 is a simulation graph of the CB shown in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 is a cross sectional view of a CB according to the presentinvention. For simplicity, the CB shown in FIG. 3 includes four layers,a ground layer 320, a voltage layer 316, and two signal trace layers 312and 324. A person of reasonable skill in the art should recognize thatthe invention might be embodied in CBs having any number of layers.

Referring to FIG. 3, a CB 300 includes a core 318 comprised of anon-conductive material, e.g., FR4. The core 318 provides structuralstrength and rigidity to the CB 300. A person of reasonable skill in theart should recognize a variety of materials for CB cores.

A reference plane (or layer) 316 is formed on the core 318. A referenceplane 320 is likewise formed on the core 318. The reference planes 316and 320 might provide a ground (e.g., GND) or a predetermined powersupply voltage (e.g., VCC) to signal traces routed on signal layers 312and 324, as explained further below. The reference planes 316 and 320might comprise 1-ounce copper. A person of reasonable skill in the artshould recognize other suitable materials for the reference planes 316and 320. A person of reasonable skill in the art should recognizewell-known methods for forming the reference planes 316 and 320 on core318, e.g., electroless or electroplating processes.

Discontinuities are formed in the reference planes, e.g., discontinuity330 formed on voltage reference plane 316. A person of reasonable skillin the art should recognize well-known methods for formingdiscontinuities 330 in the reference plane 316, e.g., standard copperetching processes that chemically etch and define patterns, planes,lines, and the like on conductive layers such as reference plane 316.

Referring to FIGS. 3 and 4A-C, one embodiment of the discontinuity 330is a split 406, that is, where a first portion 402 is separated from endto end from a second portion 404 of the reference plane 416. Anotherembodiment of the discontinuity 330 is as a slot 408 (FIG. 4B). Yetanother embodiment of the discontinuity 330 is as a cutout 410 (FIG.4C).

Referring to FIG. 3, discontinuities, e.g., discontinuity 330, in thereference planes 316 and 320 allow a signal to be cross-routed from afirst signal layer 312 to a second signal layer 324 without beingshorted through the reference planes 316 and 320. Although thediscontinuity 330 is shown only on reference plane 316, a person ofreasonable skill in the art should recognize that any number ofdiscontinuities is possible in any number of reference planes, e.g.,planes 316 and 320.

A dielectric barrier layer 326 is formed on the reference planeincluding the discontinuity, e.g., reference plane 316. The barrierlayer 326 prevents electrical shorts across the discontinuity 330. Thedielectric barrier layer 326 comprises a non-conductive epoxy material,e.g., 1060 pre-preg or a liquid curable epoxy. A person of reasonableskill in the art should recognize other suitable materials for thedielectric barrier layer 326. A person of reasonable skill in the artshould recognize well-known methods for forming the dielectric barrier326 on the reference plane 316.

The dielectric barrier layer 326 is opened on one side of thediscontinuity 330 to expose a portion of the reference plane 316. Theopening 328 might have a variety of shapes depending, e.g., on theprocess or type of equipment (e.g., laser) used to create it. Theopening might be slotted or created using vias. A person of reasonableskill in the art should recognize a variety of processes and equipmentto create the opening 328 in a variety of well-known shapes includingdrilling blind or buried vias, laser drilling vias or slots, orphotodefining openings on dielectric materials such as curable liquidepoxies or solder masks.

A conductive layer 332 spanning the length and width of thediscontinuity 330 is formed on the dielectric layer 326. The conductivelayer 332 includes a first portion 338 connecting the conductive layer332 to the reference plane 316 through the opening 328. A second portion336 bridges or spans the discontinuity 330. A third portion 334 extendsacross the reference plane 316 on another side of the discontinuity 330as shown in FIG. 3. Unlike the first portion 338, the third portion 334remains disconnected from reference plane 316. In other words, theconductive layer 332 is electrically connected to the reference plane316 at one end through the opening 328.

The conductive layer 332 increases AC signal coupling between the signallayers and underlying reference planes. The conductive layer 332,therefore, improves signal strength over a broad frequency spectrumwithout requiring additional components, e.g., stitching capacitors. Theconductive layer 332 minimizes EMC and SI problems relaxing CB signalrouting constraints, improving CB surface area usage, and reducingcomponent cost.

The conductive layer 332 might be a carbon material, e.g., graphite orcarbon black. A person of reasonable skill in the art should recognizeother suitable materials for the conductive layer 332 includingconductive materials not necessarily including carbon. The conductivelayer 332 might be deposited on the CB using a variety of well-knowncommercial processes, e.g., processes available to deposit carbonmaterials to enhance adhesion for electroless copper plating of vias.The conductive properties of carbon along with the ability to apply themin very thin layers (e.g., <1 mil thick), lend themselves to improvecoupling across plane discontinuities as described herein.

A pre-preg epoxy layer 314, the signal layers 312 and 324, and soldermask 340 and 342 complete the CB stack up. A person of reasonable skillin the art should recognize well-known methods of forming the pre-preglayers 314, the signal layers 312 and 324, and the solder masks 340 and342.

FIG. 5 is a graph of simulation results 500 for the CB shown in FIG. 3.Referring to FIGS. 3 and 5, the graph lines show results of usinggraphite for the conductive layer 332. The discontinuity 330 is 20 milswide by 2300 mils long. The reference plane 316 is made of 1-ouncecopper. The reference plane 316 and the ground plane 320 are separatedby a core 318 being 1.6 mils wide of 1060 FR4 pre-preg.

The graph lines in FIG. 5 are bounded on the one end, by simulationresults of a signal trace 502 with no underlying discontinuity and, onthe other end, by a signal trace 504 with a 20-mil discontinuity but noconductive layer 332 added to the CB. The signals traces 506, 598, and510 refer to traces over a 20-mil discontinuity with a conductive layer332 interposed as shown in FIG. 3. Trace 506 refers to a conductivelayer 332 having its third portion 334 extend over the discontinuity by5 mils. Trace 508 refers to a conductive layer 332 having its thirdportion 334 extend over the discontinuity by 10 mils. And trace 510refers to a conductive layer 332 having its third portion 334 extendover the discontinuity by 20 mils.

The simulation results shown in FIG. 5 indicate that a CB including aconductive layer 332 increases the signal strength across thediscontinuity by 10-15% over a 100 to 4,000 MHz frequency band. A personof reasonable skill in the art should understand the results shown inFIG. 5 are merely exemplary.

Having illustrated and described the principles of our invention, itshould be readily apparent to those skilled in the art that theinvention can be modified in arrangement and detail without departingfrom such principles. We claim all modifications coming within thespirit and scope of the accompanying claims.

1. A method of manufacturing a circuit board, comprising: defining adiscontinuity in a voltage reference plane; applying a carbon materialto the discontinuity such that the carbon material extends from one endof the discontinuity to another end of the discontinuity, the carbonmaterial being electrically connected to the reference plane at the oneend.
 2. The method of claim 1 wherein defining a discontinuity includesdefining at least one of a split, a slot, and a cutout.
 3. The method ofclaim 1 comprising: applying a dielectric barrier to the voltagereference plane; and opening the one end of the discontinuity throughthe dielectric barrier.
 4. The method of claim 3 wherein openingincludes: opening one end of the discontinuity with a laser slot; andfilling the laser slot with the carbon material.
 5. The method of claim3 wherein opening includes: opening one end of the discontinuity withvias; and filling the vias with the carbon material.
 6. The method ofclaim 1 wherein applying a carbon material includes applying graphite.7. The method of claim 1 wherein applying a carbon material includesapplying carbon black. 8-24. (canceled)